Embodiments of the invention relate generally to integrated circuit (IC) devices. More particularly, the subject matter disclosed herein relates to a probe tip structure for testing an array of solder bumps of an integrated circuit and for testing an integrated circuit.
Testing products with very large arrays of solder bumps require probes that are durable, accurate and reliable. Achieving good (low resistance, repeatable) electrical probe contact with a solder bump requires some degree of penetration of the bump by the probe tip in order to bypass any surface oxide. As shown in FIG. 1, the present state of art is to force rigid cylindrical probe tips 10 deposited on a ceramic or organic substrate (not shown) into the surface of tin (Sn) solder bumps 15. The cylinders are generally formed from copper (Cu) electroplating through a resist mask, after which the resist is stripped, and nickel (Ni) and gold (Au) layers are plated over the Cu.
An array of probe tips cannot be made to be perfectly planar and parallel to the solder bumps on the device to be tested. One common way to overcome this challenge is to have some compliance built into the design of the probe. Examples of compliant probes are cantilever, cobra, buckling beam as well as some membrane or elastomeric probes. However, compliant probes take up too much space due to their spring structure. Additionally, compliant probes are more expensive and complex, and also have limited current carrying capability.
Rigid probes such as thin film interposer (TFI) and probe on substrate (POS) were developed to enable testing of high power products that have a large number of closely spaced solder balls. Rigid probes overcome planarity variations by using a high force to deform the highest solder bumps enough so all bumps make electrical contact. The amount of force required can be controlled by making the probe tips smaller in diameter, minimizing the hardness of the solder bumps and improving control of solder bump and probe tip planarity. In other words, a rigid probe structure relies on compliance of the solder bumps themselves.
Referring back to FIG. 1, a problem with probe tips 10 of the present state of art is that after testing several wafers, the contact resistance begins to rise, eventually obscuring some of the desired test results. Contact resistance rises partially due to the accumulation and oxidation of Sn particles 20 which cling to probe tips 10 and decrease the surface of the probe tips 10 that contacts the solder. To restore probe tip 10 to its original low contact resistance value, probe tip 10 is subjected to a chemical cleaning process to remove Sn particles 20. Unfortunately, this procedure is lengthy and can damage probe tip 10 or other parts of the assembly such as the substrate. The process also uses dangerous chemicals.
In order to increase efficiency and safety, a laser cleaning process was introduced to ablate Sn particles 20 from probe tips 10. This process is relatively fast and does not cause damage to probe tip 10. However, the results of the laser cleaning process are not as good as the chemical clean. Consequently, laser cleaned probes need to be cleaned more often than chemically cleaned probes. During the laser clean, the temperature of the probe tip 10 can exceed the melting point of Sn, such that the Au and Sn form alloys at relatively low temperatures. While the Sn melting temperature is much lower than that of Au or Cu, by the time enough energy is applied to evaporate the Sn particles 20, there is sufficient energy to alloy the Sn and the Au or Cu. In some cases, these alloys can cause the Sn particles 20 to become more firmly attached to the Au or Cu. These residual Sn particles 20 accrue additional particles more easily than a clean surface and shorten the time between cleanings.